Claim and Claim Adjustment Expense Reserves (International - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - International - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Casualty	$ (17)	$ 9	$ (29)
Property	17	(16)	(21)
Energy and Marine	(19)	(12)	(18)
Specialty	27	12	19
Healthcare and Technology	(12)	(2)	(9)
Total pretax (favorable) unfavorable development	$ (4)	$ (9)	$ (58)